Don't be yoked with an unbeliever
"Do not be yoked together with unbelievers. For what do righteousness and wickedness have in common? Or what fellowship can light have with darkness? What harmony is there between Christ and Belial? What does a believer have in common with an unbeliever? What agreement is there between the temple of God and idols? For we are the temple of the living God. As God has said: "I will live with them and walk among them, and I will be their God, and they will be my people" (2 Corinthians 6:14-16).
God's people are warned against spiritually mixed marriages; unbelievers will lead them to sin
"And when you choose some of their daughters as wives for your sons and those daughters prostitute themselves to their gods, they will lead your sons to do the same" (Exodus 34:16).
In Ezra's time many did intermarry. This led to much sin, and Ezra confessed the guilt of God's people (Ezra 9:1-15)